Africa (Afrikagf)

AFRICA, the name of a continent representing the largest of the three
great southward projections from the main mass of the earth's
surface. It includes within its remarkably regular outline an area,
according to the most recent computations, of 11,262,000 sq. m.,
excluding the islands.1 Separated from Europe by the Mediterranean
Sea, it is joined to Asia at its N.E. extremity by the Isthmus of
Suez, 80 m. wide. From the most northerly point, Ras ben Sakka, a
little west of Cape Blanc, in 37 deg. 21' N., to the most southerly
point, Cape Agulhas, 34 deg. 51' 15'' S., is a distance approximately
of 5000 m.; from Cape Verde, 17 deg. 33' 22'' W., the westernmost
point, to Ras Hafun, 51 deg. 27' 52'' E., the most easterly
projection, is a distance (also approximately) of 4600 m. The length
of coast-line is 16,100 m. and the absence of deep indentations of
the shore is shown by the fact that Europe, which covers only
3,760,000 sq. m., has a coast-line of 19,800 m.

I. PHYSICAL GEOGRAPHY

The main structural lines of the continent show both the east-to-west
direction characteristic, at least in the eastern hemisphere, of the
more northern parts of the world, and the north-to-south direction
seen in the southern peninsulas. Africa is thus composed of two
segments at right angles, the northern running from east to west, the
southern from north to south, the subordinate lines corresponding in
the main to these two directions.

Main Geographical Features.—The mean elevation of the continent
approximates closely to 2000 ft., which is roughly the elevation of
both North and South America, but is considerably less than that of
Asia (3117 ft.). In contrast with the other continents it is marked
by the comparatively small area both of very high and of very low
ground, lands under 600 ft. occupying an unusually small part of the
surface; while not only are the highest elevations inferior to those
of Asia and South America, but the area of land over 10,000 ft. is
also quite insignificant, being represented almost entirely by
individual peaks and mountain ranges. Moderately elevated tablelands
are thus the characteristic feature of the continent, though the
surface of these is broken by higher peaks and ridges. (So prevalent
are these isolated peaks and ridges that a special term
[Inselberg-landschaft] has been adopted in Germany to describe this
kind of country, which is thought to be in great part the result of
wind action.) As a general rule, the higher tablelands lie to the
east and south, while a progressive diminution in altitude towards
the west and north is observable. Apart from the lowlands and the
Atlas range, the continent may be divided into two regions of higher
and lower plateaus, the dividing line (somewhat concave to the
north-west) running from the middle of the Red Sea to about 6 deg. S.
on the west coast. We thus obtain the following four main divisions
of the continent:—-(1) The coast plains—-often fringed
seawards by mangrove swamps—never stretching far from the
coast, except on the lower courses of streams. Recent alluvial flats
are found chiefly in the delta of the more important rivers.
Elsewhere the coast lowlands merely form the lowest steps of the
system of terraces which constitutes the ascent to the inner
plateaus. (2) The Atlas range, which, orographically, is distinct
from the rest of the continent, being unconnected with any other area
of high ground, and separated from the rest of the continent on the
south by a depressed and desert area (the Sahara), in places below
sea-level. (3) The high southern and eastern plateaus, rarely falling
below 2000 ft., and having a mean elevation of about 3500 ft. (4) The
north and west African plains, bordered and traversed by bands of
higher ground, but generally below 2000 ft. This division includes
the great desert of the Sahara.

The third and fourth divisions may be again subdivided. Thus the high
plateaus include:—(a) The South African plateau as far as about
12 deg. S., bounded east, west and south by bands of high ground
which fall steeply to the coasts. On this account South Africa has a
general resemblance to an inverted saucer. Due south the plateau rim
is formed by three parallel steps with level ground between them. The
largest of these level areas, the Great Karroo, is a dry, barren
region, and a large tract of the plateau proper is of a still more
arid character and is known as the Kalahari Desert. The South African
plateau is connected towards the north-east with (b) the East African
plateau, with probably a slightly greater average elevation, and
marked by some distinct features. It is formed by a widening out of
the eastern axis of high ground, which becomes subdivided into a
number of zones running north and south and consisting in turn of
ranges, tablelands and depressions. The most striking feature is the
existence of two great lines of depression, due largely to the
subsidence of whole segments of the earth's crust, the lowest parts
of which are occupied by vast lakes. Towards the south the two lines
converge and give place to one great valley (occupied by Lake Nyasa),
the southern part of which is less distinctly due to rifting and
subsidence than the rest of the system. Farther north the western
depression, sometimes known as the Central African trough or
Albertine rift-valley, is occupied for more than half its length by
water, forming the four lakes of Tanganyika, Kivu, Albert Edward and
Albert, the first-named over 400 m. long and the longest freshwater
lake in the world. Associated with these great valleys are a number
of volcanic peaks, the greatest of which occur on a meridional line
east of the eastern trough. The eastern depression, known as the East
African trough or rift-valley, contains much smaller lakes, many of
them brackish and without outlet, the only one comparable to those of
the western trough being Lake Rudolf or Basso Norok. At no great
distance east of this rift-valley are Kilimanjaro—with its two
peaks Kibo and Mawenzi, the former 19,321 ft., and the culminating
point of the whole continent—and Kenya (17,007 ft.). Hardly
less important is the Ruwenzori range (over 16,600 ft.), which lies
east of the western trough. Other volcanic peaks rise from the floor
of the valleys, some of the Kirunga (Mfumbiro) group, north of Lake
Kivu, being still partially active. (c) The third division of the
higher region of Africa is formed by the Abyssinian highlands, a
rugged mass of mountains forming the largest continuous area of its
altitude in the whole continent, little of its surface falling below
5000 ft., while the summits reach heights of 15,000 to 16,000 ft.
This block of country lies just west of the line of the great East
African trough, the northern continuation of which passes along its
eastern escarpment as it runs up to join the Red Sea. There is,
however, in the centre a circular basin occupied by Lake Tsana.

Both in the east and west of the continent the bordering highlands
are continued as strips of plateau parallel to the coast, the
Abyssinian mountains being continued northwards along the Red Sea
coast by a series of ridges reaching in places a height of 7000 ft.
In the west the zone of high land is broader but somewhat lower. The
most mountainous districts lie inland from the head of the Gulf of
Guinea (Adamawa, &c.), where heights of 6000 to 8000 ft. are
reached. Exactly at the head of the gulf the great peak of the
Cameroon, on a line of Volcanic action continued by the islands to
the south-west, has a height of 13,370 ft., while Clarence Peak, in
Fernando Po, the first of the line of islands, rises to over 9000.
Towards the extreme west the Futa Jallon highlands form an important
diverging point of rivers, but beyond this, as far as the Atlas
chain, the elevated rim of the continent is almost wanting.

The area between the east and west coast highlands, which north of 17
deg. N. is mainly desert, is divided into separate basins by other
bands of high ground, one of which runs nearly centrally through
North Africa in a line corresponding roughly with the curved axis of
the continent as a whole. The best marked of the basins so formed
(the Congo basin) occupies a circular area bisected by the equator,
once probably the site of an inland sea. The arid region, the
Sahara—the largest desert in the world, covering 3,500,000 sq.
m.—extends from the Atlantic to the Red Sea. Though generally
of slight elevation it contains mountain ranges with peaks rising to
8000 ft. Bordered N.W. by the Atlas range, to the N.E. a rocky
plateau separates it from the Mediterranean; this plateau gives place
at the extreme east to the delta of the Nile. That river (see below)
pierces the desert without modifying its character. The Atlas range,
the north-westerly part of the continent, between its seaward and
landward heights encloses elevated steppes in places 100 m. broad.
From the inner slopes of the plateau numerous wadis take a direction
towards the Sahara. The greater part of that now desert region is,
indeed, furrowed by old water-channels.

The following table gives the approximate altitudes of the chief
mountains and lakes of the continent:—